Sedimentation process for flocculated dispersions

ABSTRACT

THE RATE OF SEDIMENTATION OF FLOCCULES FROM DISPERSION MAY BE ACCELERATED BY INTRODUCING IN SAID DISPERSION A SMALL AMOUNT OF INSOLUBLE PARTICLES. ECH OF THESE PARTICLES ADSORBS ONTO THE PROTECTIVE COLLOID LAYER OF MORE THAN ONE FLOCCULE AND THROUGH &#34;BRIDGING&#34; MECHANISM ACTS TO HOLD THOSE FLOCCULES TOGETHER IN AGGREGATES WHICH PRECIPITATE AT A RATE MUCH FASTER THAN THE FLOCCULES INDIVIDUALLY. THE PROCESS OF THIS INVENTION IS PARTICULARLY USEFUL DURING THE WASHING STEPS OF PHOTOGRAPHIC SILVER HALIDE EMULSION PREPARATIONS EMPLOYING FLOCCULATION METHODS.

United States Patent 3,778,275 SEDIMENTATION PROCESS FOR FLOCCULATED DISPERSIONS Edward G. Denk, Waltham', Mass, assignor to Polaroid Corporation, Cambridge, Mass. No Drawing. Filed Dec. 28, 1971, Ser. No. 213,100 Int. Cl. G03c 1/02 U.S. C]. 96-94 13 Claims ABSTRACT OF THE DISCLOSURE The rate of sedimentation of floccules from dispersion may be accelerated by introducing in said dispersion a small amount of insoluble particles. Each of these particles adsorbs onto the protective colloid layer of more than one floccule and through a bridging mechanism acts to hold those floccules together in aggregates which precipitate at a rate much faster than the floccules individually. The process of this invention is particularly useful during the washing steps of photographic silver halide emulsion preparations employing flocculation methods.

BACKGROUND OF THE INVENTION Field of the invention This invention is concerned with a process for accelerating the precipitation of floccules in dispersion, and more particularly, a process for accelerating the precipitation of floccules during the preparation of photosensitive silver halide dispersions wherein extraneous soluble impurities are removed by the flocculation method.

Description of the prior art Flocculation and sedimentation has long been known to colloid chemists as an effective method for separating solids from a colloidal dispersion. In general, the term flocculation denotes the formation of clusters or aggregates of the suspended particles, referred to as floccules or flocs. The sedimentation, or precipitation of the floccules is the settling thereof that takes place as a consequence of the destabilization and aggregation of the colloidal particles during flocculation and the fact that the larger floccules move through the suspending medium more rapidly than the smaller unflocculated particles. See Alexander, J. Colloid Chemistry, vol. VI, Reinhold Publishing Co., New York (1946), pp. 782-799.

The flocculation of dilute suspensions and others carrying relatively large individual particles is best aocomplished by subjecting them to the enveloping action of a polymeric protective colloid. The stabilizing force's holding the particles in colloidal suspension are removed or rendered negligible during flocculation so that the particles coalesce upon contact. The protective colloid then envelops and holds these coalesced particles together into cluumps, or floccules, having sufficient diameter to precipitate. A polymeric protective colloid is therefore defined for the purposes of this application as a polymer which exhibits the above-described enveloping action.

The flocculation and sedimentation process has found many applications, but is of particular importance in connection with the manufacture of silver halide emulsions for use in photography. Generally, silver halide emulsions are prepared by reacting a water-soluble silver salt, such as, for example, silver nitrate, with at least one watersoluble halide such as, for example, potassium bromide, sodium bromide, potassium iodide or sodium iodide in an aqueous solution comprising a protective colloid, for example, gelatin. The dispersion of silver halide thus formed also contains water-soluble salts, produced by the double decomposition reaction, and usually excess reagents. It has been generally found desirable in modern wash techniques to separate the silver halide from such water- 3,778,275 Patented Dec. 11, 1973 soluble salts and reagents by employing various flocculation methods. It is common in the photographic art to include the precipitation of floccules within the general term flocculation. However, in the claims appended hereto, the term flocculating and precipitating have the above-described definitions, i.e. :flocculating means the forming of floccules and precipitating denotes the settling of those floccules.

One flocculation method employed comprises having present in the silver halide dispersion, at least after its formation, an acid-coagulable derivative of gelatin, and lowering the pH to bring about the coagulation and precipitation of the acid-coagulable derivatives which carry the silver halide salt crystals with them as floccules, leaving the soluble salts in solution for removal by decantation. See for example, Yutzy et al. US. Pat. Nos. 2,614;- 928; 2,614,929; and 2,728,662; and Roth US. Pat. No. 3,118,766.

Another method consists of flocculating the silver halide dispersion by adding to the dispersion a flocculating agent such as, for example, an inorganic sulfate or a polymer, allowing the sedimentation of the floccules thus formed and then removing the soluble impurities left in solution by decantation. See for example, Kinkel et al. US. Pat. No. 3,178,294; Kelly et al. US. Pat. No. 3,218,169 and Schaller et al. US. Pat. No. 3,360,373.

ilt is generally desirable to redisperse and reprecipitate the above-described floccules once they have been separated from the reaction mixture, particularly in connection with a subsequent rinsing or washing step or remov ing any additional salts or reagent which may remain.

mentioned rinsing step is repeated more than once, as is usually the case.

BRIEF SUMMARY OF THE INVENTION It has now been found that the rate of settling of floccules is greatly increased when unflocculated particles which are insoluble in the dispersion medium are introduced into the flocculated dispersion to adsorb onto the surface of the protective colloid envelope surrounding each floccule. By adsorbing onto the surface of more than one floccule, these insoluble particles act as bridges to hold the floccules together into more massive aggregates which settle faster than the original floccules themselves.

In one aspect of the present invention, the addition of a soluble silver salt such as, for example, AgNO to a flocculated dispersion such as, for example, a flocculated silver halide photographic emulsion, containing an excess of halide ion, or the addition of a soluble halide such as, for example, KBr, to a flocculated dispersion containing an excess of silver ion, or the addition of equimolar amounts of both the silver salt and the halide to a flocculated dispersion with or without an excess of either ion, leads to faster settling of the floccules.

Alternatively, an unflocculated emulsion such as, for example, a Lippmann emulsion, may be added to the flocculated dispersion to obtain the same improved settling effect.

It is therefore an object of this invention to provide novel processes for increasing the rate of sedimentation of floccules from a suspending medium.

A further object is to provide novel processes for manufacture of silver halide photographic emulsions.

Other objects of the invention will in part be obvious and in part appear hereinafter.

DETAILED DESCRIPTION OF THE INVENTION In the practice of the present invention, a small quantity of insoluble particles is made available after the formation of the above-mentioned floccules. The addition of these particles may be made during the initial precipitation or a subsequent reprecipitation. It has been found that the particles chosen must be insoluble in the dispersion medium and have the ability to adsorb onto the surface of the protective colloid envelope formed around the floccules. A particle so adsorbed onto the surface of the floccule retains its ability to adsorb onto the surface of one or more additional floccules in similar manner. By reason of this mutual adsorption onto the protective colloid surfaces of a plurality of floccules, the insoluble particle acts as a bridge to hold the floccules onto which it has adsorbed together to form an aggregate of floccules which is precipitated from the suspending medium at a rate much faster than if said aggregate had not been formed.

The sedimentation behavior of a flocculated dispersion is dependent upon a number of factors understood by those skilled in the colloid art. See the above-mentioned Alexander, Colloid Chemistry, vol. VI, Reinhold Publishing Co., New York (1946), pp. 788-791 and Parfitt, Dispersions of Powders in Liquids, Elsevier Publishing Co., Ltd., London (1969), pp. 186-190. Basically, it is known that large dense particles will fall most rapidly, and fine low density particles will tend to remain in suspension, but the behavior of those particles in any intermediate state will be dependent upon the nature of the structure in dispersion, the ratio of liquid to solid in dispersion, the viscosity of the liquid medium, etc., as much as on the particle size and density of the individual particles. Consequently, the practice of the present invention provides a novel means for providing a larger and more segregated, and therefore a generally faster-settling, particle in a flocculated dispersion; however, it is to be realized that this effect may be maximized or minimized by operator control over the above-mentioned factors according to principles well-established in the art.

It is not necessary that the above-described additional insoluble particles be the same as those encapsulated within the floccule. It is, however, necessary that said particles be insoluble in the suspending medium and have th ability to adsorb onto the surface of the protective colloid envelope of the floccule. Furthermore, it is apparent that said particles should be chemically and physically compatible with the polymeric protective colloid employed and with the ultimate utilization thereof.

The above-mentioned insoluble particles preferably comprise crystals of an insoluble salt made available as the result of adding a small amount of a compound containing the anion or cation of said salt to a flocculated dispersion which contains an excess of the cation or anion, respectively. Alternatively, salt crystals may be added directly to a flocculated dispersion, preferably in the form of an unfiocculated dispersion of said salt crystals in a suitable medium such as, for example, gelatin. Other methods of incorporating these particles into the flocculated dispersion during the settling process will be obvious to those skilled in the art. Clearly, however, this invention is directed to any process employing said adsorbed insoluble particles, however incorporated into the flocculated dispersion, whereby an increased rate of sedimentation is obtained.

As briefly outlined hereinbefore, the process of this invention is particularly useful in the preparation of photographic silver halide emulsions. In a typical preparation of this type silver halide crystals are prepared by reacting a water-soluble silver salt, such as silver nitrate, with at least one water-soluble halide, such as ammonium, potassium or sodium bromide, preferably together with a corresponding iodide, in an aqueous solution of a protective colloid such as a colloidal gelatin solution; digesting or ripening the dispersion at an elevated temperature, to provide increased crystal growth; washing the resultant dispersion to remove undesirable reaction products and residual water-soluble salts by employing any of the various flocculation systems and wash procedures adapted to effect removal of undesired components, for example, the procedures described hereinafter; after-ripening the dispersion at an elevated temperature in combination with addition of gelatin and various adjuncts, for example, chemical sensitizing agents and the like; all according to the traditional procedures of the art, as described, for example, in Neblette, C. B., Photography, Its Materials and Processes, 6th ed., 1962.

The silver halide crystals may be prepared in the gelatin protective colloid by any of the normal methods for the preparation of silver halide dispersions, such as, for example, by introducing, or jetting, a stream of an aqueous solution of silver nitrate and a stream of an aqueous solution of at least one halide, preferably potassium or sodium bromide, together with potassium or sodium iodide, into a constantly agitated solution of gelatin contained within a suitable reaction vessel. Alternatively, the gelatin solution may be combined with one of the reactans in the reaction vessel and the other reactants may be introduced with stirring. After the silver halide has formed, any ripening desirable for increasing crysal size, etc., may be effected.

Subsequent to ripening, the solution is cooled, preferably to a temperature of 30 to 35 C., and flocculation of silver halide dispersion is effected, preferably by adding a flocculating agent such as, for example, sodium sulfate, or by adding an acid such as, for example, sulfuric acid, if an acid-coagulable derivative of gelatin was used in the making stages. The precipitate thus formed may then be separated from the liquid reaction medium by any of several techniques, the simplest being decantation of the supernatant liquid with the precipitate remaining in the original reaction vessel.

The precipitate may then be subjected to a redispersion and reprecipitation procedure, known generally as a Wash procedure, to remove any undesirable reagents or byproducts remaining on the floccules such as, for example, soluble alkali nitrate, excess soluble halide and, in a process using ammonia salts, excess ammonia. The emulsion, once prepared, is preferably coated on an impervious support, such as cellulose triacetate or the like, for use in a photographic film unit. Unless removed or greatly reduced by the above washing procedure, alkali nitrate remaining in the emulsion will tend to crystallize out when the emulsion is dried and may adversely affect its physical properties. Likewise, ammonia remaining in the emulsion will cause excessive fog and a large excess of soluble halide will decrease considerably the sensitivity of the finished emulsion. It is in connection with the abovedescribed flocculation and washing procedure that the process of the present invention finds particular application.

In this embodiment of the present invention, the addition of the above-mentioned insoluble particles, preferably silver halide crystals, may occur during the initial flocculation step after the silver halide floccules have been formed, or during any subsequent reprecipitation of said floccules, or both, depending on the floccule settling rate involved and the time available. The term flocculated dispersion used in this description and in the appended claims identifies any of the above dispersions of floccules in a suspending medium. It is to be noted, however, that large amounts of the polymeric protective colloid, such as may be found during the initial flocculation step, are preferably avoided during the addition of these particles since the protective colloid acts to envelop these additional particles, thereby decreasing their necessary adsorbing qualities. For the above reason, and since the floccules generally precipitate at a faster rate during the initial flocculation procedure, the process of the present invention is preferably practiced only during one or more of the above-described reprecipitation steps conducted for washing purposes. 7

In a typical wash procedure employing the present invention the wash water is maintained at a sufficiently cold temperature to keep the fioccules from melting, for example, less than 30 C., and is added to the reaction vessel containing the separated precipitate. The resultant mixture is then agitated for a period of time thereafter sufiicient to redisperse the fioccules completely in the wash water. A quantity of additional silver halide crystals, preferably from 0.1% to 1% of the silver halide concentration in dispersion is added with continual agitation to the dispersion of fioccules thus formed.

The technique for introducing these additional silver halide crystals into the flocculated dispersion may be varied according to the particular emulsion preparation process employed. 1

In general, the progress of washing is preferably monitored by analytical tests such as, for example, conductivity or resistivity measurements, and the washing itself is usually carried to an empirically determined end-point. Since it is usually desirable to have some controlled excess of halide in the finished emulsion, an excess of halide is present throughout the washing process and, preferably, washing is continued until this excess halide has been reduced to about the amount desired for the after-ripening and finishing of the emulsion. If washing is carried beyond this point for special reasons, more halide may be added at the conclusion of washing.

Accordingly, a preferable procedure for introducing the additional silver halide crystals in one embodiment of the present invention is to introduce a stream of an aqueous solution of the soluble silver salt, e.g. AgNO into the agitated flocculated dispersion to combine with the excess halide present therein and thereby form additional silver halide crystals. This procedure is advantageous since the same equipment and reactants are utilized as during the making stage and all that is required of an operator is to activate a flow valve for a designated period of time.

Alternatively, it is possible to prepare an emulsion with, or wash an emulsion until, the silver ion is actually in excess of the bromide, because silver ion combines with gelatin to form what is loosely termed silver gelatinate. For example, a gelatin-silver bromide emulsion of average composition can be washed until about 0.4 percent of the total silver is in combination with gelatin instead of with bromide. In such cases, it may be considered desirable to introduce an aqueous solution of the halide, e.g. KBr, into the agitated flocculated dispersion to combine with sa d silver ion excess and thereby form the desired additional silver halide crystals. A still further alternative would be the addition of equal molar amounts of both AgNO- and KBr to the flocculed dispersion. The additional silver halide crystals themselves may also be added to the flocculated dispersion, such as by adding an unflocculated silver halide emulsion, for example, a Lippmann emulsion, to the flocculated dispersion. Lippmann emulsions are known to those in the photographic art as extremely fined-grained AgBr emulsions whose crystals are submicroscopic in size.

The process of the present invention, when employed in silver halide emulsion preparations such as those described above, may result in precipitation times for silver halide fioccules which may be from one-quarter to one fifth the time required for conventional processes, and in some cases, this precipitation time may decrease to as much as one-tenth the conventional time depending on the process and materials employed. Accordingly, the practice of this invention may result in a time savings of from 60-90 minutes per silver halide emulsion if three wash steps are employed. Time savings of this magnitude 6 have tremendous economic significance commercial applications.

After the fioccules have been washed in the above manner, the preparation of the emulsion may be continued in the conventional manner, e.g. the fioccules may be formulated into a photographic emulsion by after-ripening and redispersion in a gelatin, or the like, matrix as outlined hereinbefore. Alternatively, the after-ripened crystals may be dried and thus prepared for storage and shipment, to be readily redispersed at any time subsequent thereto.

It has been found that any small amount of the additional silver halide crystals remaining entrapped in the emulsion after washing does not interfere. with the photographic employment of the invention.

The present invention is further illustrated by the following non-limiting examples:

EXAMPLE I A solution of 553 gms. of water, 113 gms. of KBr, 2 gms. of KI, and 6 gms. of the trimellitic anhydride derivative of gelatin disclosed in Roth US. Pat. No. 3,118,766, was prepared and brought to a temperature of 65 C. The first of three dump silver solutions, each comprising 34 gms. of AgNO, dissolved in 323 gms. of water and maintained at a temperature of 55 C., was slowly added to the solution with stirring. Approximately 25 minutes after the addition of the first dump silver solution, the second dump silver solution was slowly added, followed 25 minutes thereafter by the third dump silver solution with continual stirring. About 30 minutes later, the solution was cooled to about 35 C. The pH was then adjusted to a value of 2.70 using 2 N H 80, to cause flocculation. The supernatant liquid was removed from the resulting precipitate by decantation.

EXAMPLE II About 1500 mls. of cold (about 8 C.) wash water were added with agitation to a precipitate prepared according to Example I above to form a dispersion of the fioccules in the wash water. A sample of this dispersion was then taken with a 30 ml. syringe at a distance approximately equal to one-half the height of the vessel. This sample was added to a long glass tube and the settling time for the fioccules recorded. This procedure was repeated to serve as control for each of the following examples.

EXAMPLE III After flocculation had taken place and approximately 1500 ml. of wash Water had been added with agitation to the silver halide fioccules prepared. in Example I above, ml. of a 0.15 N KBr solution (0.02 mole of KBr per mole of silver halide in dispersion) were added to the dispersion of fioccules thus formed. After a pause of approximately 30 seconds, an equal molar amount of AgNO (80 ml. of a 0.15 N AgNO solution) was also added to the dispersion. After a pause of about seconds, a sample was taken from the center of the dispersion as in Example II above, placed in a long glass tube and the settling time recorded. This first wash procedure was repeated three separate times.

EXAMPLE IV After each of the three first washes of Example III were accomplished and the supernatant wash medium decanted, a second 1500 mls. of wash water was added to the precipitate with agitation to form a second wash dispersion. To this dispersion, 20 mls. of a 0.15 N KBr solution (0.005 mole of KBr per mole of silver halide in dispersion) were added. After a 30 second pause, an equal molar amount of AgNO was added (20 mls. of 0.15 N AgNO solution), followed by a 90 second pause. A sample was taken from the dispersion, placed in a long 7 glass tube and the settling time recorded as in Examples 11 and III above.

EXAMPLE V After each of the second washes of Example IV, the entire procedure of Example IV was repeated and designated as a third wash.

Table I below summarizes the dramatic reductions in the settling times obtained for each of the procedures in Examples III-V above over the conventional settling times obtained in Example II. The original procedure for each example is designated A, the first repeat is designated B, and the second repeat is designated C. All times are in minutes:

TABLE I Example (first (second (third Total wash) wash) wash) time Example IIA, minutes 5. 6. 7 5. 7 17. 4. A, minutes 3. 0 1. 2. O 6. 5 Percent decrease in settling 40 78 65 62 Example 1113, minutes 6. 0 6. 4 5. 8 18. 2 13, minutes 2.8 1. 8 3. 0 7.6 Percent decrease in settling time 53 72 48 59 Example 110, minutes 6. 2 7. 5 7. 3 21. 0 0, minutes 3. 0 2. 0 l. 2 6.2 Percent decrease in settling time 52 73 84 70 Average percent decrease in settling time 48 74 66 64 The following example illustrates a preferred embodiment of the disclosed invention wherein the silver halide dispersion is made by jetting, rather than dumping as in Example I above, the solution of silver nitrate into the halide solution. Subsequent to the flocculation of the silver halide dispersion, the silver nitrate solution is rejetted or back-added" into the flocculated emulsion to effect an increased rate of floccule sedimentation.

EXAMPLE VI 55.7 gms. of KBr, 1.2 gms. of KI and about 4.5 gms. of trimellitic anhydride derivatized gelatin may be dissolved in 331.2 g'ms. of H 0, and the resultant solution brought to a temperature of about 63 C. and a pH of 6.3, essentially as described for Example I above. Then 14 liters of a 0.0256 N AgNO solution (a total of 61.2 gms. AgNO is jetted into the above-described halide and gel solution with continual agitation. At the end of the jetting period, the resultant dispersion is cooled immediately and flocculated at 35 C. by lowering the pH to a value of 2.7 with 2 N H SO When the dispersion temperature reaches 20 C., the agitator is stopped, the floccules are allowed to settle and the supernatant liquid is removed by decantation.

The flocculated dispersion is then divided into two portions, a test portion and a control portion. The control portion is washed by employing the conventional wash procedure of Example II above, i.e. wash water added with agitation and then floccules allowed to settle.

Wash water is added to the test portion, agitation is continued for about two minutes thereafter, and then, while maintaining agitation, another 100 mls. of the 0.0256 N AgNO solution (a total of about 0.4 gms. of AgNO is rejetted or back-added? to the dispersion over a period of about 2.5 minutes. The temperature of the AgNO solution is preferably maintained at less than 30 C. After this AgNO solution has been completely added, agitation is continued for an additional 1.5 minutes and then stopped, allowing thee floccules to settle.

Following the conventional procedure as specified for the control portion results in an average floccule settling time of from 40 to 60 minutes. In contrast, when the procedure of the present invention (the test portion) is followed, floccule settling times of from 9-14 minutes are possible.

The silver emulsions employing the process of this invention may be coated onto various types of rigid or flexible supports, for example, glass, paper, metal, polymeric films of both the synthetic types and those derived from naturally occurring products, etc. Especially suitable materials include paper; aluminum; polymethacrylic acid methyl and ethyl esters; vinyl chloride polymers; polyvinyl acetals; polyamides such as nylon; polyesters such as the polymeric films derived from ethylene glycolterephthalic acid; and cellulose derivatives such as cellulose acetate, triacetate, nitrate, propionate, butyrate, acetatepropionate, or acetate-butyrate.

The light-sensitive material of the photographic emulsions comprises a compound of silver, for example, one or more of thee silver halides of which silver chloride, silver bromide and silver iodide are examples. The preferred silver halide emulsion comprises a silver iodobromide emulsion. It will be understood that preferred silver halide emulsions of varying halide concentration may be advantageously employed.

The emulsions of the present invention may be chemically sensitized by any of the accepted procedures. For example, the emulsions may be digested with naturally active gelatin, or sulfur compounds can be added such as, for example, those described in U.S. Pats. Nos. 1,574,- 944, 1,623,499, and 2,410,689.

The emulsions may also be stabilized with the mercury compounds of for example U.S. Pats. Nos. 2,728,663, 2,728,664 and 2,728,665.

The emulsions may also be optically sensitized with cyanine and merocyanine dyes as described in for example U.S. Pats. Nos. 1,846,301, 1,846,302, 1,942,854, 1,990,507, 2,112,140, 2,165,338, 2,493,747, 2,493,748, 2,503,776, 2,519,001, 2,666,761, 2,743,900, 2,739,149 and 2,739,964.

The emulsions may also contain speed-increasing compounds of the quaternary ammonium type as described in, for example, U.S. Pats. Nos. 2,271,623, 2,288,226, and 2,334,864; and of the polyethylene glycol type, for example, those described in U.S. Pat. No. 2,708,162.

Where desired, suitable antifogg'ants, restrainers, accelerators, preservatives, coating aids, and/or stabilizers may be included in the composition of the emulsions.

Hardening agents such as inorganic agents providing polyvalent metallic atoms, especially polyvalent aluminum or chromium ions, for example, potash alum,

and chrome alum [K Cr (SO -24H O] and organic agents of the aldehyde type such as formaldehyde, glyoxal, mucochloric acid, etc., the ketone type such as diacetal, and the quinone type, may be incorporated in the emulsions according to procedures well known in the art.

The term photosensitive and other terms of similar import are herein employed in the generic sense to describe materials possessing physical and chemical properties which enable them to form usable images when exposed to actinic radiation.

The term gelatin as utilized herein is intended to signify the art-accepted designation of the photographic colloid binder derived from collagen and comprising a proteinaceous substance. The term is intended to include both acid and alkali process gelatins.

The term gelatin is also intended to include any other product substantially identical therewith, as for example, where such product is produced synthetically.

Since certain changes may be made in the above process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A process for preparing photosensitive silver halide which comprises the steps of:

reacting a water-soluble silver salt and at least onewater-soluble halide in an aqueous solution comprising a flocculatable protective colloid to form a colloidal dispersion of silver halide crystals;

flocculating said colloidal dispersion to form a flocculated dispersion comprising floccules of silver halide crystals enveloped in said protective colloid, said floccules precipitating from said flocculated dispersion leaving undesired components in solution;

introducing into said flocculated dispersion a distribution of insoluble silver halide crystals which mutually adsorb onto the protective colloid surfaces of a plurality of said floccules to hold said floccules together as aggregates which precipitate at a faster rate than said floccules individually, thereby decreasing the time required for the precipitation of said floccules; and

removing the silver halide crystals of said precipitated floccules from the solution containing said undesired components.

2. A process as defined in claim 1 wherein said protective colloid is gelatin.

3. A process as defined in claim 1 wherein said watersoluble silver salt is silver nitrate.

4. A process as defined in claim 1 wherein said watersoluble halide is a bromide, iodide and/or chloride.

5. A process as defined in claim 1 wherein said insoluble silver halide crystals are introduced into said fiocculated dispersion by adding an unflocculated silver halide dispersion thereto.

6. A process as defined in claim 1 wherein said insoluble silver halide crystals are introduced into said flocculated dispersion by adding a water-soluble silver salt to a fiocculated dispersion containing an excess of halide ion to form said silver halide crystals therein.

7. A process as defined in claim 1 wherein said insoluble silver halide crystals are introduced into said tloccualted dispersion by adding a water-soluble halide to a floccnlated dispersion having an excess of silver ion to form said silver halide crystals therein.

8. A process as defined in claim 1 wherein said insoluble silver halide crystals are introduced into said flocculated dispersion by adding to said flocculated dispersion an equimolar amount of both a water-soluble silver salt and a water-soluble halide to form said silver halide crystals therein.

9. A process as defined in claim 1 wherein said floccules are precipitated, removed from said solution, and redispersed in cold water at least one time before said floccules are contacted with said insoluble silver halide crystals.

10. A process as defined in claim 1 wherein the silver halide crystals of said floccules are redispersed in a gelatin matrix and coated on a support.

11. A process for preparing a photosensitive silver halide emulsion which comprises the steps of:

reacting a first quantity of silver nitrate with an excess quantity of at least one water-soluble halide in an aqueous fiocculatable gelatin solution to form a colloidal dispersion of silver halide crystals having an excess of halide ion;

fiocculating said dispersion to form a first fiocculated dispersion which comprises floccules of silver halide crystals enveloped in gelatin, said floccules precipitating from said fioeculated dispersion leaving undesired components in solution;

removing said precipitated floccules from said solution;

redispersing said removed floccules in cold water to dissolve soluble impurities from said floccules, thereby forming a second flocculated dispersion which retains said excess of halide ion;

adding a second smaller quantity of silver nitrate to said second flocculated dispersion, said second quantity of silver nitrate providing silver ion to combine with said excess halide ion to form a distribution of insoluble silver halide crystals in said second flocculated dispersion,

said insoluble silver halide crystals acting to hold precipitating floccules of said second fiocculated dispersion together as aggregates by mutually adsorbing onto the gelatin surfaces of a plurality of said floccules,

said aggregates precipitating at a faster rate than said floccules individually, thereby decreasing the time required for the precipitation of floccules from said second flocculated dispersion;

removing said precipitated floccules from said Water retaining said soluble impurities; and

redispersing the silver halide crystals of said floccules in a gelatin matrix.

12. A process as defined in claim 11 which further comprises repeating said redispersion step, said silver nitrate addition step and said floccule removal step at least once before said silver halide crystals are dispersed in said gelatin matrix.

13. A process as defined in claim 11 wherein said halide is bromide, iodide and/ or chloride.

References Cited UNITED STATES PATENTS 2,005,837 6/1935 Arens 96-94 2,146,938 2/1939 Clans 96-114.7 3,415,650 12/ 1968 Frame 96-94 3,482,982 12/ 1969 Miyata 9694 3,600,167 8/1971 Judd 96-94 FOREIGN PATENTS 4,329,405 12/ 1968 Japan 96-94 635,841 4/1950 Great Britain 9694 895,613 5/ 1962 Great Britain 96-94 OTHER REFERENCES Nernst, W., Theoretical Chemistry, 1923, Macmillan, London, pp. 501-502.

American Pharmacy, Lyman, R. A. et a1., 1945, I. B. Lippincott Co., Phila., pp. 142-145.

Textbook of Biochemistry, West, E. S. and Todd, W. R., third edition, 1961, The Macmillan Co., New York, p. 95.

NORMAN G. TORCHIN, Primary Examiner A. T. SURO PICO, Assistant Examiner US. Cl. X.R. 96-1 14.7 

